Active Galaxies With Double-Peaked Emission LinesActive Galaxies With Double-Peaked Emission LinesA large part of my work on active galactic nuclei focuses of the dynamics of the line-emitting gas, and in particular on double-peaked emission lines, which are the signature of an accretion disk around a supermassive black hole. All the details of this story are on the double-peaked emission-line page. In summary, over 20 radio-loud AGNs with broad, double-peaked Balmer lines are known today. The data for the best studied objects make a strong case that the double-peaked lines come from the other parts of the accretion disk and disfavor alternative explanations. My work now concentrates on following the variability of double-peaked emission lines and using it as a tool to study the structure and behavior of AGN accretion disks.
A significant fraction of quasars and Seyfert galaxies show narrow absorption lines superposed on their broad UV resonance lines (C IV, Si IV, N V, Ly-alpha). These absorption lines are usually attributed to an outflowing wind. The role of this low-velocity wind (v < 5,000 km/s) in the overall scheme of things is poorly understood, though. It could be related to the much faster wind thought to be responsible for the broad-absorption lines (BALs) seen in the spectra of a much smaller fraction of luminous quasars. I have been studying this phenomenon for the past few years, in collaboration with the Penn State quasar absorption-line group. we stumbled on this subject by accident when Rajib Ganguly found intrinsic absorption lines in the spectra of six quasars he was using to study intervening absorbers. He has since changed directions and has devoted most of his time on this project. He has searched for intrinsic absorption lines in the spectra of nearby quasars observed with the HST and managed to determine empirically which quasars do and do not hots narrow, intrinsic absorption lines. We are now following up these results by looking for variability of narrow, intrinsic absorption lines in quasars. we are using the HST to observe the nearby ones and the Hobby-Eberly telescope for the more distant ones.
Broad, double-peaked Balmer lines have also been found very recently in a few LINER galaxies. At least three objects, NCC 1097, Pictor A, and M 81, show transient double-peaked emission lines. A few additional objects NGC 440, NGC 4203, and NGC 4579, also exhibit double-peaked emission lines which may be transient. This underscores the connection between LINERs and radio-loud AGNs with double-peaked emission lines, It also raises the question of what is the geometry and dynamics of the accretion flows in accretion-powered LINERs. This question is particularly relevant in the context of recent suggestions for the structure of accretion flows at very low accretion rates (advection-dominated accretion flows, or ADAFs). Thus I have undertaken an observational effort, in collaboration with Aaron Barth, to obtain spectra of LINERs at high spatial resolution to measure the profiles of their broad emission lines and study the properties of the line-emitting gas. I am also involved in a similar project, with Luis Ho, to compare the properties of the central engines of LINERs as inferred from their broad emission lines and from their spectral energy distributions.
Independently of the above programs I am using Chandra to zoom in on the central engines of LINERs in the X-ray band. The high spatial resolution afforded by the Chandra mirrors allows us to dissect composite systems and study their constituents. In collaboration with Joe Shields, Ed Moran, and Jules Halpern, have observed two LINERs so far with the goal of investigating whether they are AGNs, starburst, or something else. One of them, NGC 4579 is undoubtedly an AGN, with a dominant central X-ray source. The other, NGC 4736 seems to be a starburst, with its nuclear X-ray source resolved into 2 dozen discrete sources, presumably X-ray binaries. In addition to its implications for the central engines of LINERs, this unexpected result is closely connected to another subject I am working on: modeling the population of X-ray binaries in starburst galaxies.
My original interest in this subject was the prospect of testing scenarios for the origin of optical double-peaked emission lines. But I very quickly got interested in the X-ray properties of AGNs and the problem of what causes the difference between radio-loud and radio quiet AGNs. My main collaborators in this work are Rita Sambruna and Christa Hasenkopf, a Penn State undergraduate student. We have carried out an "archival survey" survey of radio-loud AGNs observed by ASCA and we have carried out new observations with ASCA and RXTE. We are finding that radio-loud and radio-quiet AGNs are systematically different in their X-ray properties, which can be used to constrain models of their accretion flows. We are currently working on observations of X-ray variability of the brightest broad-line radio galaxies. We are also eagerly awaiting XMM observations of broad-line radio galaxies with which we hope to study the profiles of the Fe K-alpha lines.
Weak-line radio galaxies (WLRGs) are a distinct subclass of radio galaxies whose distinguishing characteristic is their very weak [O III] emission lines. They were identified in the past few years in optical surveys of radio source counterparts. In our survey of the X-ray properties of radio-loud AGNs (with Rita Sambruna; see previous paragraph), we discovered that WLRGs stand out because of their flat X-ray spectra and low X-ray luminosities. A Chandra observation of one such object, Hydra A, confirms this result by isolating the nucleus and allowing a measurement of its true X-ray luminosity. with Karen Lewis, we are now evaluating the hypothesis that the central engines WLRGs are similar to those of LINERs by studying their optical spectra.